The present invention relates to a process for regenerating hydrochloric acid from pickling plants, in which iron chloride in the spent pickling acid is thermally decomposed into iron oxide and gaseous hydrochloric acid.
In metallurgical technology for the manufacture of steel products pickling represents an essential process step. In particular hydrochloric acid and sulfuric acid as well as other acid mixtures can be used as pickling media. Because of various circumstances, partly connected with the attainable quality of the final product, partly also with the fact of complete regenerability, pickling with hydrochloric acid or mixtures containing hydrochloric acid has gained increased importance in the last 30 years. The action of the acid resides in the dissolution of mill scale layers which are formed on the steel surface by preceding processes such as rolling, annealing etc. This takes place according to the following chemical reaction: EQU FeO+2HCl.fwdarw.FeCl.sub.2 +H.sub.2 O (1)
Accordingly, during pickling there takes place a consumption of acid (HCl) up to a point where the solution is saturated with iron chloride and can no longer be used for pickling.
It has been found that the consumed pickling acid and in particular the iron chloride contained therein can be decomposed by a thermal process, resulting on the one hand in the formation of iron oxide and wherein on the other hand hydrochloric acid is recovered which can be returned to the pickling process. This proceeds according to the following reaction: EQU 2FeCl.sub.2 +2H.sub.2 O+0.5O.sub.2.fwdarw.Fe.sub.2 O.sub.3 +4HCl (2)
Two processes have gained significance for this step of thermal decomposition:
(a) the spray roasting process in which the consumed pickling acid including the iron chloride is sprayed into an empty reactor directly heated by burners, resulting in the formation of a fine dusty iron oxide, PA1 (b) the fluidized bed process in which the solution is injected into a fluidized bed reactor which contains a bed of spherical iron oxide particles which are maintained in suspension by the burner gases and the fluidization air, where a coarsely particulate iron oxide is formed.
Due to various side reactions undesirable gaseous side products which frequently entail poisonous pollutants and which by conventional technology can only be removed with difficulty or at great technological cost, may form in both processes.
Amongst these pollutants are the compounds NO and NO.sub.2 (jointly denoted as oxides of nitrogen, NO.sub.x) which on the one hand may be formed by the combustion process itself from atmospheric nitrogen, and on the other hand can be formed from nitrogen compounds added to the pickling bath, for example inhibitors.
A further pollutant is chlorine which, in the form of molecular chlorine (Cl.sub.2) is formed in the aforesaid processes by oxidation of HCl according to the so-called Deacon equilibrium. EQU 2HCl+0.5O.sub.2.fwdarw.Cl.sub.2 +H.sub.2 O (3)
The equilibrium constants of these homogeneous gas reactions are well known and are, for example, at
Temperature (K) log Kp 500 0.9 600 0.7 700 1.9 800 2.8
From this, it can be seen that the equilibrium at lower temperatures tends predominantly towards the right of the equation even though on the other hand the reaction kinetics at such temperatures are too slow in order to bring about a substantial chlorine formation. At temperatures of about 700K such as corresponds more or less to the temperature of the flue gas from a spray roasting reactor, the chlorine concentration may be calculated as follows: EQU P.sub.C12 =Kp*(P.sub.HC1).sup.2 *(P.sub.O2).sup.0.5 /P.sub.H2O
At an HCl content of e.g., 5% and an O.sub.2 content of 3.5% as well as an H.sub.2 O content of 45%--this corresponds to a typical composition of the reactor waste gas--there may be calculated therefrom a content of Cl.sub.2 of about 35 ppm or 110 mg/m.sup.3. These amounts of chlorine can vary according to the conditions, the oxygen surplus being a special deciding factor which frequently must be kept high in order to attain a predetermined oxide quality for fluidization.
The chlorine once formed can be removed only with difficulty from the waste gases. This step is, however, unavoidable since e.g., the technical regulations for air purity only permit a content of 5 mg/m.sup.3. In the context of chlorine reduction, the washing with sodium thiosulfate for example forms part of the state of the art: EQU Na.sub.2 S.sub.2 O.sub.3 +4Cl.sub.2 +5H.sub.2 O.fwdarw.2NaHSO.sub.4 +8HCl (4)
This manner of chlorine removal requires, however, expensive gas scrubbers and a corresponding consumption of chemicals. In addition, effluents are formed which have to be disposed of.